Method and apparatus for molding composite articles

ABSTRACT

A method and apparatus for molding composite articles includes a lower frame ( 14 ) with a contoured flange, a semi-flexible lower skin ( 16 ) and a semi-rigid upper mold half ( 18 ). The frame has a contoured flange ( 24 ) for drawing a vacuum on a flange ( 62 ) of the lower skin. The upper mold half is formed from a skin ( 20 ) having a trusswork to make the skin semi-rigid. Pressure sensors ( 66 ) are mounted in the mold cavity to sense pressure change of the vacuum with respect to ambient and to control the injection rate of the resin in the cavity.

FIELD OF THE INVENTION

The present invention relates to a method of molding and moldingapparatus for use in the molding of composite articles. Moreparticularly, the invention relates to a method and apparatus for use inresin transfer molding.

BACKGROUND OF THE INVENTION

Resin transfer molding (RTM) is a process in which dry fiberreinforcement is loaded into a mold cavity. The surfaces of the moldcavity define the ultimate configuration of the article beingfabricated. Resin is injected under pressure or drawn under vacuum intothe mold cavity to saturate the fiber reinforcement. After the resinatedfiber reinforcement is cured, the finished article is removed from themold.

Recently, RTM molding has been performed in a rigid cavity or lower moldand a skin forming the upper mold. Such a molding process is disclosedin United Kingdom Patent Application 2,319,205A. The flexible upper moldskin is typically made from a composite material. The upper skin isformed over an inverted male mold pattern. Then, a calibration layer ofsheet wax defining the mold cavity is laid over the upper skin and abolster skin is formed over the calibration layer. The exposed face ofthe bolster skin is shrouded with a frame. The pattern is separated andthe calibration layer is removed. The upper skin is then used as part ofthe upper mold half. However, the skin has a short life expectancy andnew skins can be replaced only by reapplying the sheet wax andrebuilding the tipper skin from a calibrated wax surface. This requiresthe tooling to be out of production for a lengthy period of timedepending upon the size of the mold. Additionally, the flow of resinoccasionally backs up as it is blown through the fiber reinforcement inthe mold cavity, thereby causing an outward deformation of the skin andloss of tolerance in the article being formed.

BRIEF DESCRIPTIONS OF THE INVENTION

The invention is directed to a novel molding apparatus, a method offorming the apparatus, and a method of using the apparatus to form acomposite article. The apparatus includes a semi-flexible skin supportedon a lower frame and a semi-rigid upper mold half. The lower frameincludes a peripheral flange which surrounds an opening for receivingthe cavity of the lower skin. The flange has a contour which is formedto mirror the corresponding structure of the skin. A vacuum is used todraw the lower skin tightly against the flange of the frame and an innerperipheral edge of the flange. The upper mold is formed by applyingcalibration sheet wax within the cavity of the lower skin. A suitabletooling surface material is applied to the sheet wax followed by acasting of additional materials to form the upper mold half. The skin isthen structurally supported by building a trusswork across the back sideof the skin to make the upper mold half semi-rigid. A peripheral flangewith vacuum ports is formed similar to the flange on the frame to permita vacuum to be drawn to draw the upper mold tightly against the lowerskin. Deflection sensors are mounted to the back sides of the upper andlower skins. The sensors are connected to a CPU to stop or slow the flowof resin if the resin pressure builds to deform the skins.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be explained in further detail with reference tothe embodiments shown in the drawings in which:

FIG. 1 is a partial sectional view and schematic of the moldingapparatus in accordance with the invention;

FIG. 2 is a perspective view of the frame and flange for the lower moldhalf in accordance with the invention;

FIG. 3 is a cross-sectional exploded view of the upper and lower moldhalves;

FIG. 4A is a cross-sectional view of the upper and lower mold halves ofthe apparatus in accordance with the invention;

FIG. 4B is a partial cross-sectional view of the upper mold half inaccordance with the invention;

FIG. 5 is a cross-sectional view of a mechanical sensor in accordancewith the invention; and

FIG. 6 is a flowchart showing methods of forming the apparatus inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an apparatus 10 for resin transfer molding (RTM)includes a lower mold half 12 having a frame 14, a lower cavity skin 16,and an upper mold half 18 having a semi-rigid mold skin 20. The skins ofthe apparatus 10 form a cavity 22 (FIG. 4A or plenum for molding anarticle of composite material.

As shown in FIGS. 2, 3 and 4, the lower flame 14 includes a supportflange 24 supported on a tubular base 26. The flange 24 is formed toextend peripherally around a cavity portion 28 of tire lower skin 16(FIG. 4). The support flange 24 has a top surface 30 which is contouredto match a peripheral flange 44 of the cavity skin 16. A pair of spacedapart seals 32, 33 or rubber gaskets extend longitudinally along the topsurface 32. A plurality of openings 34 for drawing a vacuum arepositioned between the seals 32, 33 and extend through the supportflange 24 to a manifold 36 formed on an underside of the flange 24. Themanifold 36 is connected by a conduit 38 to a vacuum pump 40 and controlunit 42. When the pump 40 is activated, the peripheral flange 44 isdrawn against the support flange 24 and the skin is drawn tightlyagainst an inner edge 46 of the support flange 24 to provide rigidity tothe skin 16. The inner edge 46 is formed to nest securely against theskin 16. The support flange 24 is formed by band lay-up on the back sideof the skin 16. The seals are installed and then resinated cloth is laidon to form a mirror image of the back side of the flange of the skin.Depending upon the size of the article being molded, it may be necessaryto provide a composite form 48 extending under a center of the cavityskin 16. The form 48 acts like a sling to support a middle portion ofthe cavity skin 16.

As shown in FIG. 3 the cavity skin 14 is formed with the perimeterflange 44 extending outwardly from the cavity portion 28. The skin 16has a front side 52 and a back side 54. The front side 52 of the cavityportion 50 has the shape of one side of the article to be formed. Thecavity skin 16 is formed on a male mold pattern 56 by a suitable mannersuch as hand lay-up, vacuum bagging, vacuum form thermoset orthermoplastic sheet. The mold pattern has the shape of the article beingformed. The front side 52 of the skin 16 is formed on the pattern. Themold also forms the flange. A pair of gaskets are placed on the portionof the mold forming the flange to form recesses on the flange of theskin to receive seals 32 of the frame as discussed below.

In most applications the molded article will have an outer layer of gelcoat. In such applications, the gel coat 56 is applied to the front sideof the cavity portion 28 of the skin. Once the gel coat 56 has cured onthe cavity portion 28, reinforcement fibers 60 are then fitted to thecavity form. The reinforcement fibers 58 may be fiberglass, aramid,carbon or synthetic fibers which are laid on the gel coat 56 in thecavity portion 28 of the skin. After the skin 16 has been fitted withthe reinforcement fibers 58, the skin is placed on the frame with theunderside of the peripheral flange 44 of the skin resting on the supportflange 24 of the frame. The inner edge 46 of the support flange 24 ofthe frame 14 extends around the back side of the cavity skin 16 wherethe cavity portion 28 begins. When a vacuum is drawn, the cavity skin 16is locked onto the flame 14 and the support flange 24 provides the skinwith great rigidity.

As shown in FIGS. 4A and 4B, the upper mold half 18 includes an upperskin 20 which is backed by a truss frame 60 having a flange 62containing another set of vacuum seals 64, 65. The skin is formed in thesame manner as described in GB 2,319,205 A. The truss frame 60 extendsacross a back side 62 of the skin 20 to make the skin semi-rigid. Theframe 60 may be formed of a suitable rigid material such as metal and ispermanently affixed by resin to the skin. The frame 60 houses a vacuummanifold 64 which extends about the flange 62 of the upper skin. Themanifold 64 is connected to the pump 40. Seals 63, 65 are mounted to theflange 62. The upper mold half 18 and lower mold half 12 have registryguides such as a dowel as known in the art to maintain the upper andlower mold halves 18, 12 in proper position. After the upper mold half18 is placed over the cavity skin 24, a vacuum is drawn on the upperhalf to lock the upper mold half on the front side of the cavity skinflange.

As shown in FIG. 1, a vacuum is created in the cavity through a port inthe center of the upper mold half 18 from either a catchpot 59 mounteddirectly on the top of the port or through a tube feeding to a resintrap as is known in the art. After the mold halves have been installedand registered, a vacuum on the flange of the lower skin is drawn bypump 63. Then, a vacuum on the cavity is drawn, typically 0.5 bar, andresin is injected from a supply of resin 51 through ports 53, 55.

As shown in FIG. 4A, pressure sensors, such as a pressure transducer 66,are mounted to both the back sides of the upper and lower mold skins fordetecting the pressure within the cavity. The sensor may be pressuretransducer 66 or a mechanical valve 68 (FIG. 1) which reacts to a changein pressure. The pressure transducer 66 has a probe which passes throughthe skin into the cavity. The pressure transducer is able to detectsmall changes in pressure of 0.1 bar or less. A suitable pressuretransducer is produced by Micron Instruments of Simi Valley, Calif. Thepressure in the cavity is less than the ambient pressure surrounding theskins. The CPU receives the signals from the sensors, and if there is apositive gain in pressure, the CPU operates a valve to slow or stop theflow of resin before there is a deflection of the skins. When one ormore of the sensors sense an increase of pressure at too high a rate orabove a threshold pressure, the CPU acts to reduce or stop the flow ofresin.

As shown in FIG. 5, the pressure sensor may be a mechanical valve 68having a housing 70 and a piston 72. A cavity pressure diaphragm 74 ismounted on one end of the piston 72. The housing is mounted to the skinby mold insert ring 75 to be in direct contact with the cavity. Onesurface of the piston 72 is in a chamber 73 which is connected to thevacuum source 63. A pilot poppet 74 is mounted opposite the piston 72.The pilot poppet 74 is connected to the pump power air supply 76 for aresin pump 78. The pilot poppet 74 is in a normally closed positionproviding control for the power supply air flowing to the pump 78. Thepiston 72 is movable through the chamber 73 to close the poppet 74.

When the cavity 22 is placed under vacuum, the pressure is exactly thesame in both the cavity 22 and the vacuum connection port which isdirectly connected to the vacuum source 63 drawing central vacuum. Sincethe pressure is the same on the cavity diaphragm 74 and in the chamber73, the piston will remain in a neutral state in which the piston 72 isnot in contact with the pilot poppet 74. If the pressure in the cavity22 increases over the vacuum source pressure, the piston 72 will move toopen the pilot poppet 74. Once the piston 72 is lifted from its neutralposition, the pilot poppet will release the air holding thepneumatically powered resin pump supply line open.

In an alternative embodiment of the pressure control apparatus, thepiston is spring biased against the pressure diaphragm. When the innermold cavity pressure is eater than the biasing force of the spring, thepiston will move to trigger the pilot poppet as above. The sensitivityof the system using this valve is limited to the biasing resistance ofthe spring. In the previous embodiment, where the vacuum pressure isapplied to one side of the valve, minimal changes in mold cavitypressure will result in movement of the piston to trigger the poppet.

When the leak signal is lost, a signal is sent to sever the resin pumpfrom its power supply air and the resin pressure intensified by theresin pump is immediately lost. This allows the cavity pressure tobecome negative again and the piston then returns to the normal positionopening the poppet. The leak signal then is restored and the resin pumppower supply is restored to allow the pump to again begin pumping. Thisoperation continues in a “closed loop” until the mold cavity is filled.

As shown in FIG. 1, the valve 68 or sensor 66 may be used with moldshaving multiple injection ports 53, 55. The injection ports 53, 55 arespaced successively inwardly from the perimeter of the mold towards thecenter exit vent 56. Resin is injected in the outermost injection pointsfirst. When the valve 68 senses a positive increase of cavity pressure,pilot pressure is sent to a diversion valve 90 to close the initialperimeter injection point and to open a succeeding inner injectionpoint(s) to further the resin flow to the center while maintaining anegative cavity pressure. The injection may also be accomplished byusing the electronic sensors 66 and CPU to control the flow.

After the injection is complete, the mold is held with the vacuum on thevacuum frame holding the cavity mold and upper mold to maintain closureof the mold halves until the resin cures within the mold cavity. Afterthe prescribed cure time has elapsed, the upper mold half is lifted offthe cavity and the molded product is removed.

As set forth in FIG. 6 in greater detail with respect to the descriptionof the apparatus, the method includes forming a semi-flexible skin on amale mold pattern for use in defining a cavity for a lower mold half. Acontoured flange for supporting the skin on a frame is then formed. Theflange has the contour and an inner surface to mate with the back sideof a flange. A next step is forming a semi-rigid upper mold half. Thesemi-rigid mold half may be formed by laying calibration wax over themold pattern and casting the skin on the calibration wax. A trussworkframe with a vacuum manifold on a flange is mounted to the back of theupper skin. The next step is to place the lower skin onto the flange ofthe frame and to place the upper mold half on top of the lower skin. Avacuum is drawn through the contoured surface of the lower skin andthrough the outer flange of the upper mold half to close the moldhalves. Finally, resin is injected into the cavity.

Additionally, a method of controlling the injection of the resinincludes mounting a pressure sensor in a portion of the cavity,generating signals indicative of the pressure at intervals during theinjection process, noting the rate of change of the pressure within thecavity, and controlling injection of resin into the cavity in responseto pressure sensed by the sensors.

Thus disclosed is an apparatus and method for RTM molding which is lessexpensive than conventional methods. The cavity of the mold can beeasily replaced at a fraction of the cost of conventional toolingmethods. The cavity can be duplicated at minimal expense, thus multiplecavity skins may be used each simultaneously. A gel coat andreinforcement fiber can be applied while other skins are moldingproducts in the mold flames and the shin can be replaced without havingto remove the mating half from production.

1. A mold for molding an article by resin transfer molding, said moldcomprising: a lower mold half having a frame and a semi-flexible skin,said frame having a flange for supporting said skin, said frame furtherhaving a manifold connected to a plurality of apertures formed in saidflange; a vacuum pump connected to said manifold for drawing a vacuumthrough said apertures on a back side of said skin; and an upper moldhalf having a surface spaced apart from a front side surface of saidskin to form a mold cavity for forming said article.
 2. The mold half ofclaim 1 further having at least one pressure sensor extending throughsaid skin into said cavity for sensing the pressure within the cavityand a controller for controlling the flow of resin in said cavity inresponse to pressure sensed in said cavity.
 3. The mold of claim 1wherein said upper mold half comprises a skin and a trusswork mounted toa back side of said skin.
 4. The mold of claim 2 wherein said flange ofsaid flame has a contoured surface adapted to conform to said back sideof a portion of said skin.
 5. The mold of claim 4 wherein said flangehas an inner peripheral edge formed to nest against a back side of aportion of said skin.
 6. An apparatus for molding an article comprising:at least one skin having a back side and a front side, said front sidedefining a portion of a mold cavity; a pressure sensor mounted to a backside of said skin and having a portion extending through said skin intosaid cavity, said sensor generating a signal indicative of pressure insaid cavity; and a control unit for controlling the injection of resininto said cavity in response to said signal generated by said sensor. 7.The apparatus of claim 6 wherein said sensor is a pressure transducer.8. The apparatus of claim 6 wherein said sensor is a valve having amembrane mounted in said cavity.
 9. A method of molding an article, saidmethod comprising the steps of: forming a skin having a peripheralflange extending around a mold cavity; placing said skin in a lowerframe; placing an upper mold having a flange on the upper surface ofsaid flange of said skin; drawing a vacuum through said lower frame on aback side surface of said flange of said skin and drawing a vacuumthrough said upper mold on an upper surface of said flange of said skin;and drawing resin through a plenum formed between said skin and saidupper mold to form the article.
 10. The method of claim 9 furthercomprising drawing a back side peripheral portion of said cavity againsta peripheral edge of said frame to provide rigidity to said skin. 11.The method of claim 9 further comprising forming said upper mold from asemi-flexible skin and forming a framework on a back side of said skinto provide rigidity to said mold.
 12. A method of controlling a moldingprocess comprising the steps of: mounting a sensor on a deformable skindefining a portion of a mold cavity; injecting resin into said cavity;sensing deformation of said skin with said sensor; generating a signalindicative of deformation; and controlling the flow of resin in responseto said signal.